1. Field of the Invention
The present invention relates to access control methods, and more particularly to an access control method in a communication system using a point-to-multipoint network structure.
2. Description of the Background Art
The recent trend toward relaxation of regulations in broadcastings and communications has facilitated fusion of the two and experiments are being conducted for two-way communications using cable television networks and the like. In a communication system using a point-to-multipoint network structure such as the cable television system, a master station assigns communication channels to secondary stations. The secondary stations communicate with the master station by using the communication channels assigned. In the polling system, which is one of the methods for assigning the communication channels, a master station asks the secondary stations whether they have transmission messages. When a large number of secondary stations are accommodated in a communication system, however, there has been a conventional problem that it takes long before a transmission message is actually sent out after being generated in a certain secondary station.
The method explained below is suggested to solve this problem to accommodate a large number of secondary stations in a communication system. That is to say, the secondary stations are divided into some groups and the communication channels are assigned to the groups. Then the master station asks the secondary stations, group by group, whether they have transmission messages.
When the method described above is applied, however, a large number of secondary stations in a certain group (hereinafter referred to as "a first group") may have transmission messages while only a small number of secondary stations have transmission messages in another group (hereinafter referred to as "a second group"). That is to say, a traffic difference occurs between the groups. Then the secondary stations belonging to the first group will require a long time before actually sending out a transmission message after it is generated. Furthermore, if the traffic on the communication channels assigned to the first group is congested in spite of the fact that the communication channels assigned to the second group are not being used, the secondary stations belonging to the first group can not communicate until a communication channel assigned to the first group becomes free. This brings about the problem that the secondary stations in the first group provide extremely lower response and throughput than those in the second group. That is to say, the method above produces the problem that the communication channels (frequencies, for example) can not be efficiently used in the communication system.